1. Field of the Invention
The present invention relates to power supplies. More specifically, the present invention relates to high voltage power supplies for free electron lasers and other devices.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Certain devices require a stable high voltage power supply for proper operation. A free electron laser (FEL) is one such device. An FEL is a device in which a beam of electrons is passed through a spatially varying magnetic field. The magnetic field causes electrons in the beam to "wiggle" and radiate energy. In this device, the high voltage power supply performs the function of accelerating electrons to provide the high energy input electron beam. More specifically, FELs and certain other devices require accurate applied voltages on the order of one part in one thousand. Otherwise the frequency and power gain of these devices change with time and produce undesirable results. Thus, several techniques are known in the art for providing such a high voltage supply. These techniques include radio frequency (RF) linear accelerators, pulse linear accelerators, transformers, and Marx banks.
In RF linear accelerators, electrons in a beam are accelerated by an oscillatory field provided by standing electromagnetic waves (microwaves) in a series of cavities. This technique works best for the electrons which enter the cavity at precisely the right time. As a consequence, the accelerated beam is a series of pulses of short duration, i.e., shorter than a microwave period. Unfortunately, the FEL is typical of those applications in which a more steady state electron flow is required to produce free electron lasing interaction on more of a steady state basis.
Pulse linear accelerators have a transformer which steps up a voltage and applies this voltage to an electron beam accelerator. The voltage is often stepped up in series so pulses of electrons are accelerated by a series of structures. Unfortunately, these devices also do not generally supply constant pulses of long duration.
It is possible to operate certain devices at lower voltages using transformers. However, transformers operating at high voltages have difficulty with respect to the voltage time product. That is, the duration at which a high voltage is sustainable is limited by the nature of the transformer. Increases in sustainable pulse lengths come at a cost in terms of transformer size and weight. Output pulse length can not therefore be arbitrarily increased.
Nonetheless, transformers have several features which allow for some degree of trade off in design. Typically the tradeoff comes down to pulse rise time versus pulse flatness. Unfortunately, certain devices, such as the FEL, are designed to operate at a specific voltage. When the transformer is initially turned on, before reaching the operating range of the device, the voltage rises through some range in which the device may experience undesirable and perhaps damaging effects. Accordingly, in FEL and other applications, it is not desirable to initiate a pulse with a long rise time as the electron beam will not be properly focused during the rise of the pulse.
A Marx bank is a set of capacitors switched from a parallel configuration, in which the capacitors are charged, to a series configuration from which the stored voltage is discharged. Essentially, a capacitor is driving the device. While this may be the currently preferred approach, there is a significant problem associated therewith. That is, as the sustainability of the output pulse is defined by the resistive-capacitive (RC) time constant of the device, long steady pulses require long RC time constants. Thus, the extent to which the supply voltage can be sustained at the desired steady state level is dependent on the capacitance of the bank. Unfortunately, as the capacitance of the bank goes up, the hazards associated with the operation of the device go up due to the relationship between the energy stored and capacitance.
Thus, there is a need in the art for a safe, stable, constant voltage, constant current power supply or modulator for operating devices, such as free electron lasers, at voltages of 100 kilovolts or more with pulse durations of tens of micro-seconds.